1. Field of the Invention
The present invention relates to nonreciprocal circuit devices, composite electronic components, and communication apparatuses incorporating the same, used in high frequency bands such as a microwave band.
2. Description of the Related Art
Recently, communication apparatuses such as cellular phones have adopted digital modulation systems of high band-utilization efficiencies. The modulation systems include a xc2xc-pi wavelength QPSK system, a CDMA system, and the like. In such communication apparatuses, linear power amplifiers are used as transmission power amplifiers. Additionally, in order to permit continuous long-hour talks over the telephones while reducing power consumption, the linear power amplifiers used in the above communication apparatuses have high efficiencies. However, the highly efficient linear power amplifiers have characteristics susceptible to changes in load impedance. In other words, the linear power amplifier can show its high efficiency only when the load impedance is fixed at a desired value. For example, when a load having great changes in input impedance level, such as an antenna, is directly connected to the linear power amplifier, the efficiency of the linear power amplifier decreases and the input/output linear characteristics are thereby deteriorated. As a result, since the power consumption in the amplifier increases, the length of time permitting communications is reduced. Furthermore, transmission waves tend to be distorted, thereby leading to the occurrence of interrupting waves between adjacent channels.
In order to solve the above problems, for example, as shown in FIG. 8, a lumped-constant isolator 30 is interposed between a linear power amplifier 20 and an antenna. The linear power amplifier 20 has a structure constituted by connecting an input matching circuit 21, a first-stage amplifying device 22, an interstage matching circuit 23, a second-stage amplifying device 24, and an output matching circuit 25. In the equivalent circuit of the isolator 30 shown in FIG. 9, three central conductors 31, 32, and 33 intersect with each other. A ferrite member 34 is disposed at the part where the three central conductors 31, 32, and 33 intersect with each other, and a DC magnetic field HDC is applied to the part. Matching capacitors C1, C2, and C3 are connected in parallel to the central conductors 31 to 33, respectively. A terminating resistor R is connected to a port P3 of the central conductor 33. Each of the central conductors 31 to 33 serves equivalently as an inductance L. In the isolator 30, the input impedance is stable regardless of changes in load impedance. That is, the isolator 30 has a function of stabilizing impedance matching by absorbing the reflection from a load. This function prevents reduction of the efficiency of the linear power amplifier 20 and deterioration of the input/output linearity. Typically, since the input/output characteristic impedance of the linear power amplifier 20 is designed to be 50 ohms, the input impedance in the isolator 30 is usually set to be 50 ohms, which is a standard value in a high frequency component.
Meanwhile, with miniaturization and weight reduction of cellular phones, the voltages of batteries used in the cellular phones have been set to be lower. The output voltages in the cellular phones are reduced to be in a range of approximately 3 to 4V. As a result, a rated operational voltage of the linear power amplifier is also set to be in the range of approximately 3 to 4V. A saturated electric power level of the linear power amplifier is determined by the operational voltage of the amplifier and the output impedance of an amplifying device such as a GaAs-FET or a silicon bipolar transistor. For example, in a linear power amplifier having a rated output power of approximately 1 W, the saturated electric power level is set to be approximately 2 W so as to obtain leeway.
However, as shown in FIG. 8, when the above low power-supply voltage is used, the output impedance of the output amplifying device 24 is in a range of approximately 3 to 10 ohms. This is much lower than the output impedance of the linear power amplifier 20, which is set to be 50 ohms as a normal value. As a result, in the above linear power amplifier 20, the output matching circuit 25 is connected to the output amplifying device 24 to convert the output impedance value of the linear power amplifier 20 into 50 ohms. However, when the low impedance level ranging from approximately 3 to 10 ohms is converted into 50 ohms, power loss due to loss in the output matching circuit 25 occurs and the width of a frequency band capable of matching is narrowed. Thus, these problems create factors that reduce the efficiency of the linear power amplifier 20 and the operational frequency bandwidth.
In addition, since a substrate on which the isolator is mounted, which is a circuit board, is formed into a thin plate, for example, there is a problem in that it is impossible to form a microstrip line at the characteristic impedance of 50 ohms with good width precision. Consequently, the isolator having the input/output impedance of 50 ohms has a problem in that matching failures occur.
Accordingly, it is an object of the present invention to provide a nonreciprocal circuit device, a composite electronic component, and a communication apparatus incorporating the same, in which there is no need to dispose an output matching circuit in a power amplifier while obtaining a good impedance matching with the power amplifier and/or a circuit board.
According to a first aspect of the present invention, there is provided a nonreciprocal circuit device including a plurality of central conductors arranged to intersect each other, a ferrite member disposed at the point of intersection of the central conductors and being adapted to receive a DC magnetic field, a plurality of ports corresponding to the plurality of central conductors, each of the ports having a matching capacitor connected to the corresponding central conductor in one of a series configuration and a parallel configuration.
With this arrangement, in the port having the matching capacitor connected in series to the corresponding central conductor, there is formed an LC series resonant circuit constituted of the inductance and the matching capacitor of the central conductor. The input/output impedance of this port can be set to be greatly lower than that of each of the ports having the matching capacitors connected in parallel to the corresponding central conductors. In other words, without adding another component used for performing impedance conversion, by connecting the matching capacitor in series to the central conductor and by changing the value of the matching capacitor, the input/output impedance can be set at an arbitrary low value.
In this case, the input/output impedance may be set to be in a range of 1 to 15 ohms according to the input/output impedance of an output-stage amplifying device of a power amplifier. With this arrangement, it is unnecessary to dispose a matching circuit used for performing impedance conversion, which is necessary in the conventional art. Accordingly, the power amplifier can be miniaturized while obtaining high efficiency and a wide frequency band.
Furthermore, with the use of the nonreciprocal circuit device in accordance with the present invention, the width of the microstrip line on a circuit board can be increased. As a result, the nonreciprocal circuit device can be mounted on the circuit board in a fixed and stable manner so that matching failures associated with the width of the microstrip line can be prevented.
In addition, the above nonreciprocal circuit device may further include a terminating resistor connected to one of the ports to form an isolator.
According to a second aspect of the present invention, there is provided a composite electronic component including the nonreciprocal circuit device and the power amplifier that are integrated with each other by connecting the nonreciprocal circuit device to an output section of the power amplifier. With this arrangement, a low-priced and compact composite electronic component having good characteristics can be obtained.
According to a third aspect of the present invention, there is provided a communication apparatus including one of the nonreciprocal circuit device and the composite electronic component described above. With this arrangement, similarly, a compact communication apparatus having good characteristics can be obtained at a low cost.